Process for producing a protective chromium layer

ABSTRACT

Process for producing a gastight and crack-free protective chromium layer for substrates composed of iron- and nickel- and titanium-based alloys by means of plasma spraying, where the chromium content in the finished layer is at least 70% by weight and a spray powder composed of three components, namely a first component composed of finely particulate chromium powder, a second composed of finely particulate powder of a nickel-based alloy and a third composed of coarsely particulate cristobalite or quartz powder as support for the first and second component, is selected.

The invention relates to a process for producing a protective chromiumlayer according to the preamble of claim 1 as well as to the use of aplasma-sprayed protective layer.

PRIOR ART

Chromium is one of the most important metals for coatings. Its very highcorrosion resistance to many aggressive media in a broad temperaturerange is comparable with that of noble metals. Depending on how they areproduced, chromium coatings have very different properties.

Three types of coatings based on chromium are known:

-   -   1. galvanic chromium layers    -   2. PVD and CVD chromium layers    -   3. chromium layers formed by high-temperature diffusion

Galvanic chromium layers are the oldest and most widely distributedlayers based on chromium. The first description of electrolyticdeposition of chromium goes back to A. C. Becquerel in 1843. In 1854, R.W. Bunsen described the deposition of chromium from hot chromium(III)chloride solution with carbon anodes and platinum cathodes. ErikLiebreichs invented chromium deposition in the chromium bath (DE 398054and DE 448526). Therein the galvanic bath consisted of CrO₃ and H₂SO₄.Heretofore almost all chromium layers have been produced according tothis method. In the process, layers of pure chromium with a thickness of<1 μm to approximately 300 μm are applied on completely differentsubstrates (metals, glasses, ceramics, plastics and even wood).Depending on layer thickness, the coating is known as decorative chromeplating (layers<5 μm) or as hard chrome plating (layer thicknesses:10-200 μm). The properties of galvanically deposited chromium layersconsist in very high hardness and brittleness, relatively weak adherenceto the substrate and a fine network of cracks at layer thicknesses>5 μm.These properties, together with the fact that chromium has a lowcoefficient of thermal expansion—well under that of the most importantmetallic substrates—limit the use of galvanic chromium layersconsiderably. Because of fine cracks, these chromium layers arebasically permeable for gaseous and liquid media, their mechanicaldurability is relatively low due to weak adherence and high brittleness,and the maximum permissible operating temperature is lower than 500° C.,even though chromium as a compact metal can withstand temperatureshigher than 1100° C. in air.

PVD and CVD chromium layers are obtained by deposition from the gasphase in the vacuum furnace. A distinction is made between purelyphysical deposition from chromium vapor (physical vapor deposition,abbreviated PVD) and deposition by means of a chemical reaction betweenchromium-containing gas and substrate (chemical vapor deposition,abbreviated CVD). Because of this chemical reaction, the CVD chromiumlayers basically have higher adherence than PVD chromium layers.However, the CVD process requires considerably higher temperatures of800-1000° C., compared with 200-500° C. for the PVD process. Bothprocesses permit the application of dense thin layers from pure chromiumor from chromium nitride (CrN). Compared with galvanic chromium layers,the PVD and especially the CVD chromium layers have very good adherenceto the substrate, but on the other hand are much more expensive thangalvanic layers and therefore are of only limited use for parts withlarge surface areas. Moreover, the maximum layer thickness is onlyapproximately 10 μm.

The coating of steels by means of thermochemical diffusion of chromiumat temperatures of 1000-1200° C., known by the term “thermal chromeplating”, comprises two different process variants, although they leadlargely to the same results: the known (DE 1905717) diffusion ofchromium from a solid phase, e.g. chromium powder, and the known gaschromizing (EP 0043742 A1) from a gas phase, e.g. CrCl₃. In bothprocesses, chromium diffuses into a steel surface as far as a depth ofapproximately 50 μm, thus forming a protective layer. This diffusionlayer has a maximum chromium concentration of 50%, together with highcorrosion resistance and high hardness of the steel surface. Since thediffusion chromium layers are actually not pure chromium layers such asgalvanic or PVD chromium layers, they also have entirely differentproperties, such as good adherence, good mechanical strength and acoefficient of thermal expansion close to that of steel. Theseproperties permit use of parts coated with diffusion chromium attemperatures higher than 800° C. Compared with layers of pure chromium,however, the high-temperature corrosion resistance of the diffusionchromium layers is much poorer than that for compact metallic chromium.The layers of pure chromium are also superior in terms of wet chemicalcorrosion resistance. Despite comparatively favorable properties ofdiffusion chromium layers, their use is only very limited, because oftheir high complexity.

Of the known chromium layers described in the foregoing, only diffusionchromium layers are suitable for use at high temperatures above 800° C.Because of their relatively low chromium content of at most 50%,however, they do not attain the desired resistance of pure chromiumlayers. Relative to the layer thickness, all known chromium layers areinadequate, since the maximum layer thickness of dense crack-freechromium layers is limited to approximately 10 μm.

From EP 2006410 A2 there is further known a thermally sprayed protectivelayer for metallic substrates, wherein the spray powder comprises atleast two components, of which the first is a silicatic mineral or rockand the second is a metal powder and/or a further silicatic mineral orrock.

Furthermore, DE 69313456 T2 describes a coating material of ceramiccomposition, wherein the applied metal layer may contain quartz glassamong other components.

Finally, WO 2003/031672 A1 discloses a spray powder composed of ceramicparticles, including quartz, and a metal powder consisting of Ni, Cr, Feand Si.

The objective of the present invention is to exploit the advantages ofmetallic chromium as a material for protective layers againsthigh-temperature corrosion without having to accept its disadvantages.The desired improvement is intended to relate to the followingproperties of chromium layers:

-   -   continuous use in air should be possible up to 1000° C.    -   the layer should be resistant to thermal shock    -   the chromium content of the layer should be at least 70%    -   good adherence to the substrate should be assured    -   high gas-tightness of the layer should be achieved by        appropriate freedom from cracks    -   layer thicknesses up to approximately 1 mm should be possible.

In particular, the object according to the present invention is to finda solution to the effect that fine-grained chromium powder can be usedwithout disadvantages, i.e. adequate adherence can be achieved despiteextensive oxidation of the fine chromium particles; high kinetic energyand thus pore-free and sound microstructure can be obtained and goodfree-flowing ability of the powder can be achieved despite fine chromiumparticles; a further object is to find additives for the chromium powderthat reduce the brittleness of the layer and increase its coefficient ofthermal expansion; finally, another object is to develop a method forheat treatment of the coated substrates that leads to a strongmetallurgical bond between layer and substrate.

This object is achieved according to the body of claim 1 by the combineduse of the plasma-spray process with the inventive powder mixture and ifnecessary a subsequent diffusion heat treatment.

The application of the chromium layers by thermal spraying according toclaim 1 permits low costs even for large parts. Moreover, it permits theapplication of quite thick layers, which would not be conceivable withknown techniques such as galvanization, PVD, CVD, gas chromizing andinchromizing.

Because of the high melting point of chromium, approximately 1900° C.,plasma spraying is optimally selected. Under the process conditionsaccording to claim 1, it permits melting of the chromium powder butprevents scorching (oxidation) thereof in the flame.

Accordingly, the inventive process provides for melting the chromiumparticles and accelerating them against a substrate. Furthermore, itrelates to creating oxidation protection both for the free-flowingpowder and for the surface of the substrate under the flame. Since thetemperature of the particles in the plasma is determined mainly by theirsize during plasma spraying, the chromium particles should besufficiently fine-grained to ensure that they will melt. On the otherhand, the use of fine-grained chromium powder means that it will be verysusceptible to oxidation, because of its large surface area. Theparticles of chromium oxide Cr₂O₃ formed in the process cannot bereduced in the plasma but instead are melted and accelerated togetherwith chromium particles against the substrate. A consequent disadvantageis that the chromium oxide hinders the adherence between metallicsubstrate and chromium, since a metallurgical bond cannot be formed.

A further disadvantage of fine-grained chromium powder consists in a lowkinetic energy of the small particles in the flame, with the consequencethat the layer microstructure formed has inadequate adherence and isneither pore-free nor sufficiently sound.

This and further disadvantages due to plasma spraying of purefine-grained chromium powder are overcome by the inventive process byvirtue of the composition of the spray powder according to claim 1.

In this connection, the powder for plasma spraying is preferablygenerated by simple dry mixing of three components:

chromium powder<20 μm (d50<10 μm): 30-50 wt %

powder of a nickel-base alloy (e.g. 80Ni20Cr)<20 μm (d50<10 μm): 5-10 wt%

cristobalite or quartz powder 50-100 μm (d50=70-90 μm): the rest

In this mixture, lightweight (2.3 g/cm³) coarse-grained cristobalitepowder functions as carrier for the heavy fine-grained chromium andnickel-chromium powders: large cristobalite particles, with a volumeamounting to more than 70% of the mixture (<30 vol % Cr+NiCr), arecovered on the surface with fine chromium-base and nickel-baseparticles. These fairly large, round agglomerates make the powderefficiently free-flowing. In the plasma, the agglomerates are heatedsufficiently at the surface that all metallic particles melt. Underthese conditions the large refractory (1720° C.) cristobalite corebasically remains solid. By virtue of its size and its weight, anagglomerate particle consisting of a cristobalite core ensheathed with amolten metallic “crust” acquires high kinetic energy in the plasmaflame. When it collides with the substrate, the following events takeplace:

The solid cristobalite core bursts into small fragments, which reboundfrom the substrate and are carried away by the gas stream. Only afraction of the original amount of cristobalite, namely approximately1-5% of the layer mass, is “also pulled in”. These cristobalite residuesthen form small uniformly distributed inclusions (<20 μm) in thefinished metallic layer. In contrast, almost the entire metallicfraction of the spray powder remains “sticking” on the substrate, thusforming a fine-structured dense layer. The large cristobalite particlesfulfill yet another advantageous function: upon colliding with thesubstrate or with inner strata, the hard and brittle grains act as akind of sandblasting material, which “strips” oxide layers (Cr₂O₃)immediately during coating. Thereby the adherence to the substrate andbetween individual strata and the layer microstructure becomes stronger,and only minimum contents of Cr₂O₃ remain. Since the nickel-base alloyhas a much lower melting temperature than chromium, it solidifies laterthan the chromium. Thus fine nickel-base lamellas are formed on thesurface of the chromium particles, meaning that hard chromium particlesin the finished layer are ensheathed by a fine “network” of softnickel-base alloy. The “network” of soft nickel-base alloy significantlyincreases the ductility of the layer. Stresses developing duringsolidification of molten chromium no longer lead to crack formation, butare dissipated by the plastic deformation of the nickel-base lamellas.

The resulting layer has the following composition:

-   -   chromium: 70-90 wt %    -   nickel-base alloy: 7-25 wt %    -   cristobalite: 1-5 wt % (3-15 vol %)

Even with an admixture of quartz powder, the finished layer containsonly cristobalite, because quartz is transformed to cristobalite in theplasma.

Since cristobalite has a very high coefficient of thermal expansion,approximately 50×10⁻⁶ K⁻¹, the coefficient of thermal expansion of thelayer reaches approximately 9-10×10⁻⁶ K⁻¹ for the layer as a whole(compared with approximately 6.2×10⁻⁶ K⁻¹ for pure chromium). This valueis already close to values for some steels, nickel-base alloys andtitanium alloys, with the result that no harmful stresses can develop inthe layer during cooling.

The adherence of the layer on iron-base and nickel-base alloys can beimproved even more by heat treatment of the coated parts. This iscarried out in the furnace at temperatures of 900° C. and higher in air.This heat treatment for up to approximately 5 hours leads to diffusionof the chromium from the layer into the substrate to approximately 5 μm.By virtue of this diffusion, the layer and the substrate are “weldedtogether”, as it were. At the same time, the heat treatment anneals outthe residual stresses present in the layer after plasma spraying.

EXAMPLES Example 1

Use in highly-stressed valves of large diesels running on heavy fueloil: corrosion protection for nickel-base alloys against aggressivefused ashes (sodium vanadate) in combination with SO₂-containing exhaustgases and temperatures up to approximately 900° C.

A powder, mixed together from 40 wt % chromium<20 μm, 10 wt %80Ni20Cr<20 μm and 50 wt % cristobalite 50-100 μm, was sprayed by meansof the Axial-3 plasma spraying system of Thermico GmbH with thefollowing parameters onto a valve disk of Nimonic 80A:

Nozzle: ⅜″

Current: 200 A (burner power: 95 kW)

Plasma gas: Argon—200 L/min, nitrogen—55 L/min, hydrogen—12 L/min

Powder gas: Nitrogen—10 L/min

Powder flow: 20 g/min

The coated valve was heat-treated at 1020° C. for one hour in air.

After the coating process and the heat treatment, the layer formed onthe surface of the valve disk was 800 μm thick, free of pores andcracks, and had the following composition:

chromium: approximately 82 vol %

80Ni20Cr: approximately 12 vol %

Cr₂O₃: approximately 3 vol %

cristobalite: approximately 3 vol %

Example 2

Use in highly-stressed pipes of garbage incineration systems: corrosionprotection for steels against chloride and sulfate ashes in combinationwith exhaust gases containing SO₂ and HCl and temperatures up toapproximately 600° C.

A powder, mixed together from 40 wt % chromium<20 μm, 10 wt %80Ni20Cr<20 μm and 50 wt % cristobalite 50-100 μm, was sprayed by meansof the Axial-3 plasma spraying system of Thermico GmbH with thefollowing parameters onto a boiler pipe of steel 37:

Nozzle: ⅜″

Current: 200 A (burner power: 95 kW)

Plasma gas: Argon—200 L/min, nitrogen—55 L/min, hydrogen—12 L/min

Powder gas: Nitrogen—10 L/min

Powder flow: 20 g/min

The coated pipe was heat-treated at 900° C. for five hours in air.

After the coating process and the heat treatment, the layer formed onthe pipe surface was 100 μm thick, free of pores and cracks, and had thefollowing composition:

chromium: approximately 82 vol %

80Ni20Cr: approximately 12 vol %

Cr₂O₃: approximately 3 vol %

cristobalite: approximately 3 vol %

Example 3

Use in highly-stressed titanium valves of racing engines: oxidationprotection for all titanium alloys and titanium aluminides attemperatures up to approximately 800° C.

A powder, mixed together from 40 wt % chromium<20 μm, 10 wt% 80Ni20Cr<20μm and 50 wt % cristobalite 50-100 μm, was sprayed by means of theAxial-3 plasma spraying system of Thermico GmbH with the followingparameters onto a valve disk and stem of Ti6Al2Sn4Zr2Mo:

Nozzle: ⅜″

Current: 200 A (burner power: 95 kW)

Plasma gas: Argon—200 L/min, nitrogen—55 L/min, hydrogen—12 L/min

Powder gas: Nitrogen—10 L/min

Powder flow: 20 g/min

After the coating process, the layer formed on the complete valvesurface was 100 μm thick, free of pores and cracks, and had thefollowing composition:

chromium: approximately 84 vol %

80Ni20Cr: approximately 12 vol %

Cr₂O₃: approximately 1 vol %

cristobalite: approximately 3 vol %

This layer on the valve stem also functions as a wear-resistant runninglayer.

1. A process for producing a gas-tight and crack-free protectivechromium layer for substrates of alloys based on iron and nickel andtitanium by plasma spraying, wherein a spray powder is selected fromthree components, a first component of fine-grained chromium powder, asecond component for improvement of the mechanical characteristics ofthe protective layer of fine-grained powder of a nickel-base alloy and athird component for improvement of the coefficients of thermal expansionof the protective layer of coarse-grained cristobalite or quartz powderas carrier for the first and second components, and wherein the chromiumcontent in the formed protective layer is at least 70 wt %, the contentof the nickel-base alloy is 7-25 wt % and the cristobalite content is1-5 wt %.
 2. (canceled)
 3. (canceled)
 4. A process according to claim 1,characterized by a subsequent heat treatment of the protective layer inair at temperatures higher than 900° C.
 5. A process according to claim1, wherein the thickness of the protective layer is selected up to 1 mm.6. The use of a plasma-sprayed protective layer according to claim 1 onsubstrates threatened by corrosion from components of internalcombustion engines, gas turbines, steam turbines, propulsion-unitcompressors or heat exchangers.